Look for your nearest friendly ammo pylon. They are easy to spot when you know what to look for due to their tallness and the picture of bullets on the side. They aren't present at every base however, so you may need to search around a bit.
Watch towers tend to always have one. Watchtowers are the multi-leveled bases with landing pads for air vehicles and a ground vehicle spawn on the bottom level.
Sunderers can also be outfitted for ammo replenishment. There is currently no easy way to tell what a sunderer is fitted with, so your best bet is to drive over and see if your ammo grows.